Magnesium batteries remain promising as possible replacements for lithium-ion batteries, with critical advantages of low cost and high earth-abundance of magnesium. Beyond economic and environmental reasons, magnesium is also desirous as an anode material because non-dendritic electrochemical behavior can be observed and magnesium offers a large theoretical volumetric capacity of 3832 mAh/cm3 (J. Muldoon, et al., “Electrolyte roadblocks to a magnesium rechargeable battery,” Energy & Environmental Science, 5, 5941 (2012) and T. D. Gregory, et al., “Nonaqueous Electrochemistry of Magnesium: Applications to Energy Storage,” Journal of The Electrochemical Society, 137, 775 (1990), each of which is incorporated by reference in its entirety). Thus the research and optimization associated with magnesium batteries have been a focus of research recently (D. Aurbach, et al., “Progress in Rechargeable Magnesium Battery Technology,” Advanced Materials, 19, 4260 (2007); H. D. Yoo, et al., “Mg rechargeable batteries: an on-going challenge,” Energy & Environmental Science, 6, 2265 (2013); E. Levi, et al., “On the Way to Rechargeable Mg Batteries: The Challenge of New Cathode Materials,” Chemistry of Materials, 22, 860 (2009); and P. Novák, et al., “Magnesium insertion electrodes for rechargeable nonaqueous batteries—a competitive alternative to lithium?,” Electrochimica Acta, 45, 351 (1999), each of which is incorporated by reference in its entirety). Notably, there are still challenges that must be overcome to make application of magnesium battery technology widespread.
A fundamental challenge associated with magnesium cathodes is the Mg2+ intercalation into host materials. The strong polarization of the small divalent Mg2+ requires shielding or some other approach to reduce the impact on the inherently slow ion diffusion (Muldoon, 2012). In this aspect, some success was encountered with the development of Chevrel phase materials, which exhibit relatively fast Mg-ion diffusion and high capacity, although at a voltage that is lower than ideal (Aurbach, 2007 and Yoo, 2013).
Alternatively, MoS2 has been a potentially viable cathode, with density functional theory (DFT) calculations predicting a maximum theoretical capacity of 223 mAh/g (S. Yang, et al., “First-principles study of zigzag MoS2 nanoribbon as a promising cathode material for rechargeable Mg batteries,” The Journal of Physical Chemistry C, 116, 1307 (2011), which is incorporated by reference in its entirety), and experimental reports demonstrating a discharge capacity of 119 mAh/g (Y. Liu, et al., “Sandwich-structured graphene-like MoS2/C microspheres for rechargeable Mg batteries,” Journal of Materials Chemistry A, 1, 5822 (2013), which is incorporated by reference in its entirety).
In addition, manganese oxides including α-MnO2, birnessite-MnO2, and hollandite-MnO2 have recently been tested, where birnessite-MnO2 materials realized capacities of 109 mAh/g, while hollandite-MnO2 cathodes showed discharge capacities as high as 475 mAh/g (Yoo, 2013; S. Rasul, et al., “High capacity positive electrodes for secondary Mg-ion batteries,” Electrochimica Acta, 82, 243 (2012); and R. Zhang, et al., “α-MnO2 as a cathode material for rechargeable Mg batteries,” Electrochemistry Communications, 23, 110 (2012), each of which is incorporated by reference in its entirety).
Vanadium-based oxides are appealing due to the ready availability of multiple valence states (V5+→V3+), offering the potential for high energy density due to multiple electrons transferred per formula unit (C. J. Patridge, et al., “Synthesis, Spectroscopic Characterization, and Observation of Massive Metal-Insulator Transitions in Nanowires of a Nonstoichiometric Vanadium Oxide Bronze,” Nano Letters, 10, 2448 (2010), which is incorporated by reference in its entirety, and A. S. Tracey, et al., Vanadium: Chemistry, Biochemistry, Pharmacology, and Practical Applications, particularly at Chapter 13, pp. 221-239, CRC Press, Florida (2007), which is incorporated by reference).
Previous electrochemical studies of vanadium oxide, V2O5, as a cathode material in magnesium-based electrolytes have shown that capacities of ˜170 mAh/g could be achieved, where the capacity was found to improve with water added to the electrolyte (L. Yu and X. Zhang, “Electrochemical insertion of magnesium ions into V2O5 from aprotic electrolytes with varied water content,” J. Colloid Interface Sci., 278, 160 (2004); P. Novak and J. Desilvestro, “Electrochemical Insertion of Magnesium in Metal Oxides and Sulfides from Aprotic Electrolytes,” J. Electrochem. Soc., 140, 140 (1993); P. Novak, et al., “Magnesium Insertion in Vanadium Oxides: A Structural Study,” Z. Phys. Chem. (Munich), 185, 51 (1994); and P. Novak, et al., “Electrochemical Insertion of Magnesium into Hydrated Vanadium Bronzes,” J. Electrochem. Soc., 142, 2544 (1995), each of which is incorporated by reference in its entirety).
In other studies the effects of vanadium oxide morphology on the electrochemistry was explored. For example, a reversible insertion of magnesium was observed with vanadium oxide nanotubes, with reported capacities of 120 mAh/g (L. Jiao, et al., “Electrochemical insertion of magnesium in open-ended vanadium oxide nanotubes,” J. Power Sources, 156, 673 (2006); L. Jiao, et al., “Mg intercalation properties into open-ended vanadium oxide nanotubes,” Electrochem. Commun., 7, 431 (2005); and L.-F. Jiao, et al., “Synthesis of Cu0.1-doped vanadium oxide nanotubes and their application as cathode materials for rechargeable magnesium batteries,” Electrochem. Commun., 8, 1041 (2006), each of which is incorporated by reference in its entirety). Thin film vanadium oxide prepared via high temperature thermal vacuum deposition has been found to deliver 150-180 mAh/g (G. Gershinsky, et al., “Electrochemical and Spectroscopic Analysis of Mg2+ Intercalation into Thin Film Electrodes of Layered Oxides: V2O5 and MoO3,” Langmuir, 29, 10964 (2013), which is incorporated by reference in its entirety).
In general, sol-gel synthetic strategies for materials preparation can lead to scale up and commercialization and thus are appropriate for the preparation of materials with possible industrial applications. An early report on the insertion of polyvalent ions used vanadium oxide aerogels prepared by ion exchange of sodium metavanadate (D. B. Le, et al., “Intercalation of Polyvalent Cations into V2O5 Aerogels,” Chemistry of Materials, 10, 682 (1998), which is incorporated by reference in its entirety). Insertion of Mg2+ into V2O5 aerogel experimentally showed that the gel prepared materials can be effective hosts for polyvalent and well as monovalent cations.
A later report of sol-gel based preparations of V2O5 used hydrogen peroxide and metallic vanadium powder as precursors (D. Imamura, et al., “Mg Intercalation Properties into V2O5 gel/Carbon Composites under High-Rate Condition,” J. Electrochem. Soc., 150, A753 (2003), which is incorporated by reference in its entirety). Thin coatings on indium-tin oxide glass were prepared and showed reversible peaks by voltammetry and sustained currents as high as 20 A/g. A sol-gel preparation of MgV2O6 from Mg(CH3COO)2, citric acid, and NH4VO3 was reported followed by extensive thermal treatment at 350° C. and 600° C. (J.-Z. Sun, “Preparation and Characterization of Cathode Material for Magnesium Cells,” Asian J. Chem., 23, 1397 (2011), which is incorporated by reference in its entirety). An initial delivered capacity of 120 mAh/g with 40 mAh/g delivered after 10 cycles was observed.
While prior cathode materials have advanced significantly in recent years, there remain challenges in the synthesis and utilization of suitable electrode materials for Group II cation-based batteries.